Contamination of a plastic bag or plastic packaging by the filling material is difficult to prevent in many packaging processes, particularly in the packaging of foodstuffs. Most problems occur primarily with packaging processes which do not include automated process steps during the filling of the packaging material with the filling material. Process steps of this nature can be seen to a large extent with chamber machines or belted chamber machines. Packaging materials used with this type of machine are normally plastic bags. Here, filling is carried out by operating personnel who must take the greatest care not to contaminate the sealing-weld surfaces. Furthermore, it must be considered that a high processing rate also needs to be achieved with the packaging of foodstuffs. Firstly, this is due to economic aspects and secondly, particularly with the packaging of foodstuffs, also due to the perishable nature of the product.
Another reason, which renders the prevention of contamination difficult, is the shape of the packaging material. Whereas with packaging machines for plastic trays an automatic filling process is technically easily implemented, the filling of plastic bags for sealing in chamber machines is difficult to automate. Also the risk that the sealing-weld surface is contaminated by the filling material, is higher with bags than with trays, whether this is with manual or automatic filling. Since the product to be packaged comes very close to the sealing-weld surface or must pass by it due to the shape of the bag, contamination of this surface is difficult to prevent. In addition the sealing-weld surfaces can be better protected by specially adapted cover devices when filling trays.
The main problem which occurs through the wetting of the sealing-weld surface by different types of material is that the ensuing sealing seam does not exhibit the desired strength of sealing or has leaks. If large product residues are present in the sealing-weld region, the molecular movements in the plastic melt during the sealing process are impeded.
The effect of product residues in the sealing weld can be explained principally based on a peel system. Due to the inclusions of product residues in the sealing weld, the seal layers are not joined together over the whole surface. Depending on the number and size of these inclusions, reduced strength of the sealing weld occurs, through to leakage. The weld has a tendency to tear open between the sealing layer and the product residues. In contrast to the peel system this is not desired here.
Not all products which are packaged present a problem for sealing and for the strength of the sealing weld. There are also unproblematic types of contamination which do not negatively affect the sealing process. Experience with this problem has shown that amongst other factors the consistency or viscosity of the filling material is decisive. The thinner the filling material, the fewer problems occur with the sealing process, because during sealing the filling material can be forced out of the fusing region by the applied sealing pressure. Strongly adhesive contaminants, highly viscous products and powder-type filling material more often cause problems when closing the package with a sealing weld. Also the composition of the filling material is relevant.
For example, meat juice very much impairs the sealing process. Since meat juice is mostly of low viscosity, it should be possible to easily force it out of the sealing zone. However, if the structure of meat and therefore also meat juice is considered, then it is found that it mainly consists of proteins. If a large number of these proteins are present in the sealing layer during the sealing process, then the macromolecules of the sealing process of the packaging material cannot form sufficient linkages. Due to the different structure of the proteins and the macromolecules of the melt, there is no or only weak bonding capability present and therefore it has a very negative effect on the strength of the sealing weld.
With contamination due to very small particles which do not wet the sealing zone over the complete area, the result can be improved by a wider sealing weld. With pronounced contamination over large areas of the sealing weld this measure usually brings no improvement. Sealing profiles are known which force the contamination out of the sealing region. They present an expedient solution. With oval, triangular or half-round profiles attempts are made to remove unwanted residues from the joining zone during sealing.
Similarly, the state of the art is to use ultrasound techniques for sealing. With ultrasonic sealing any contaminants in the sealing weld are forced out of the sealing-weld region by the vibrations occurring during sealing. In this respect the direction of movement is carried out orthogonal to the sealing plane. Ultrasound is employed today in sealing tools of flow pack machines, butt-ended bagging machines as well as for closing cups and tubes. In the field of evacuated packages or packages with modified atmospheres the use of ultrasound techniques is very complex and expensive.